1. Field of the Invention
This invention relates to a polarization control element for obtaining a linearly polarized single longitudinal mode laser beam and to a solid state laser which is provided with such polarization control element in its resonator and generates a linearly polarized single longitudinal mode laser beam.
2. Description of the Related Art
As disclosed, for instance, in Japanese Unexamined Patent Publication No. 62(1987)-189783, there has been known a solid state laser in which a solid laser crystal doped with a rare earth element such as neodymium (Nd) is pumped with a laser beam emitted from a semiconductor laser or the like. In this type of solid state laser, in order to obtain a laser beam having a shorter wavelength, it is widely practiced to convert the wavelength of a laser beam emitted from the solid laser crystal to a second harmonic by a crystal of a nonlinear optical material disposed in the resonator of the solid state laser.
There has been a demand for this type of laser as well as lasers of other types to be able to generate a linearly polarized single longitudinal mode laser beam. However when an optically isotropic crystal like a YAG crystal is employed as the solid laser medium, a linearly polarized laser beam cannot be obtained. Conventionally, a Brewster's plate for polarization control is disposed in the resonator in order to obtain a linearly polarized laser beam or a Brewster's plate and an etalon are disposed in the resonator in order to obtain a linearly polarized single longitudinal mode laser beam.
However there has been known the fact that imperfectness in surface accuracy and a slight deviation from the Brewster's angle of the optically polished surface of the Brewster's plate as well as scatter on the surface and/or inside of the Brewster's plate can result in large loss of light introduced into the resonator. Also scatter on the surface and/or inside of the etalon can result in loss of light introduced into the resonator though not so large as that in the Brewster's plate. Such loss of light deteriorates the oscillation efficiency of the solid state laser.
Especially when both the Brewster's plate and the etalon are disposed in the resonator, the manufacturing cost of these optical elements and the cost for adjustment of these optical elements are high and add to the manufacturing cost of the solid state laser.
As a polarization control element which can overcomes the aforesaid problems, there has been known a polarization control element disclosed in U.S. Pat. No. 5,502,738. The polarization control element is formed by angle-cutting a birefringent crystal, that is, cutting the crystal so that its light incident face and light emanating face are at an angle to the optical axis of the crystal, and adjusting the thickness and the reflectivities of the faces of the crystal so that the crystal can function as a Fabry-Perot etalon.
When a light ray enters the polarization control element, the ordinary ray and the extraordinary ray of the light ray are separated from each other. When the polarization control element is disposed in a resonator of a laser, one the ordinary ray and the extraordinary ray is selected according to the position of the resonator mirrors and is caused to oscillate, whereby a linearly polarized laser beam emanates from the resonator. At the same time, the polarization control element which functions also as an etalon selects the wavelength and makes the laser beam a single longitudinal mode.
The polarization control element of a birefringent crystal generates a loss of light introduced into the resonator smaller than that of the Brewster's plate, and accordingly, the solid state laser provided with such a polarization control element can oscillate at a high efficiency.
However the solid state laser with such a polarization control element sometimes comes to generate a laser beam polarized in a direction normal to the original polarizing direction in a high-order transverse mode in a certain temperature range when the temperature of the resonator and/or the pumping semiconductor laser changes while the solid state laser is generating a linearly polarized laser beam in a single longitudinal mode or a single transverse mode.
When the solid state laser comes to generate a laser beam in a high-order transverse mode, the linear polarization properties deteriorate and especially when the wavelength of the laser beam is converted by a wavelength conversion element, the output of the wavelength-converted beam lowers.